Hot Neutron Star Explains Superburst Frequency

A neutron star is one of the few possible endpoints of stellar evolution. A neutron star is formed from the collapsed remnant of a massive star after a supernova, and has a mass between 1.35 to about 2.1 solar masses, with a corresponding radius between 20 and 10 km (they shrink as their mass increases), 30 000 to 70 000 times smaller than the Sun.

Astronomers have finally found out why the surfaces of certain neutron stars are significantly hotter than previously expected.

NSCL professor and co-author of the new study to be published in the Astrophysical Journal, Hendrik Schatz and his team used mathematics and computer codes to build a new theoretical thermometer to explain the superbursts - the observed frequency of ultra-violent explosions - that sometimes ignite on such stars' surfaces.

"This is the first model that goes into some reasonable detail about the nuclear physics that occur in the crusts of accreting neutron stars," said Schatz.

Superbursts emanate from binary systems in which a neutron star orbits a companion star. When the two stars get close enough together, a steady rain of material is sucked away from the companion star onto the surface of the neutron star.

Since a neutron star is so dense - on Earth, one teaspoonful would weigh a billion tons - the companion star material that reaches the neutron star surface is strongly compressed and heated. Eventually nuclear reactions trigger an explosion that burns through the surface layer of accumulated material, resulting in a burst of X-rays clearly detectable by ground- and space-based instruments.

Repeating every few hours to days, X-ray bursts are fusing hydrogen and helium into a mixture of elements that is itself potentially reactive. In contrast, superbursts occur when, after many months, the accumulated "ashes" produced in the X-ray bursts ignite in a different, even more dramatic nuclear explosion.

The outpouring of X-rays that results is 1,000 times as energetic as a standard X-ray burst. One superburst, which lasts only for a few hours, releases as much energy as the sun will radiate in a decade.

Together with colleagues at Los Alamos National Laboratory and the University of Mainz in Germany, Joint Institute for Nuclear Astrophysics, (JINA)-affiliated NSCL scientists built the most accurate model to-date of the crusts of accreting neutron stars.
The team calculated that reactions in the stars' crusts release 10 times more heat than indicated by earlier models.

According to the research team, this newly discovered heat would help them to reconcile the work of theorists and experimentalists who study neutron stars.
Prior to this study, theoretical astrophysicists predicted that superbursts should occur every ten years or so. Now, according to the new calculation, theorists can explain why the gigantic explosions should occur every three or four years.